Environ. Sci. Technol. 1992,26,625-626
A Novel Photocatalytic Method for Detoxification of Cyanide Wastes Deepak Bhakta,t Shyam S. Shukla,t M. S. Chandrasekharaiah,"and John L. Margrave Materials Science Research Center, Houston Advanced Research Center, 4800 Research Forest Drive, The Woodlands, Texas 77381
The safe disposal of aqueous cyanide wastes (RCRA Code FXXX type hazardous wastes; annual production in the United States is about 5-6 billion gallons) is a challenging waste management problem in the 1990s (1-7). The alkaline chlorination process (the most widely used unit process for the detoxification of cyanide wastes at present) is less than satisfactory (1-6). As the complexed cyanides (e.g., iron, cobalt, and nickel cyanides) are not affected by this process, they invariably end up in the sludges formed during the chlorination process (1-5). The RCRA Act of 1987, Section 3004, bans the land disposal of solid wastes containing cyanides (2, 7). Thus, in order to comply with the EPA regulations of cyanide disposal, it has become necessary to develop an alternate to alkaline chlorination for the safe disposal of cyanide wastes. It is an established fact that semiconductors produce electron-hole pairs at their surface when excited by suitable radiation (8-12). These photogenerated electron-hole pairs can act as good reductants or oxidants in aqueous phase. In fact, photodissociation of water at the surface of TiO, microelectrodes in the presence of solar radiation has been reported (9,21,22). The photogenerated holes act as strong oxidants and the versatility of these photocatalyzed reactions at the TiOz microelectrodes has been investigated in detail by Bard and Frank (13). The oxidation reactions carried out through these photogenerated holes are different from the direct photochemical reactions using the same radiation. In this study, we have examined the use of UV-irradiated TiOz microelectrodes to oxidize the complexed cyanides. Complexed cyanides such as nickel, iron, and cobalt cyanides are the most oxidation resistant cyanide species in solution (1-3). In this study, therefore, oxidation of ferricyanide with irradiated TiO, sol was examined and the results are presented. As the photocatalytic oxidation of cyanides takes place principally at the TiO, semiconductor-solution interface (9, I I ) , it is necessary to have large surface/volume ratios for the Ti02. In this work, this was achieved by keeping the titania in the form of a colloidal suspension or sol. A typical experiment consisted of preparing a dilute M) by adding the transparent TiO, sol (- 1-1.15 X requisite amount of titanium tetrachloride into cold water and then stabilizing the prepared sol in alkaline solution (pH = 8-11) by adding -1 wt % poly(viny1 alcohol) (13000-23000 Da). To this sol were added weighed amounts of potassium ferricyanide to make the concentration of ferricyanide to 1.0 mM. This mixture was then taken in a fused-silica cuvette and exposed to UV light (a 4-W mercury lamp). The concentration of ferricyanide as a function of time was inferred from the experimentally measured absorption peak of ferricyanide at 420 nm. Results are summarized in Tables I and I1 and in Figure
-
1.
The detoxification reactions of cyanide waste have to be carried out in an alkaline medium in order to prevent the formation of highly toxic cyanogen chloride and hydrocyanic acid gases. Therefore, all our photocatalytic oxidation studies of ferricyanide were carried out in solu+Permanentaddress: Department of Chemistry, Lamar University, Beaumont, TX 77710. 0013-936X/92/0926-0625$03.00/0
Table I. Photocatalytic Degradation of Ferricyanide" at Titania Sol Surface with a 4-W UV lamp: Effect of pH % ferricyanide left
time, h
pH = 8
pH = 10
pH = 12
0 3 6 9
100 70
100
100 59 53 28
54 23 7
39
21
"Initial ferricyanide concentration was the same in all cases (1 mM). Table 11. Photocatalytic Degradation of Ferricyanide at the Titania Sol Surface by Solar Radiation" time, h 0.0
1.5 3.25
% ferricvanide left fused-silica cell Corning glass cell
100 0
100 54 0
"The pH = 10 and the initial ferricyanide concentration was -1 mM.
tions of pH = 8-12. The results presented in Table I show that the photodegradation of complexed cyanide was complete within 10 h at all three pH values. The highest rates were observed at pH = 10. The results presented in Figure 1show clearly that the ferricyanide concentration was reduced to almost zero in the solution containing titania sol while there was hardly any change in the ferricyanide content in the absence of titania sol. This is clear proof that the photodegradation of ferricyanide is catalyzed by the titania sol and not by a direct absorption of the UV light. The absorption edge of the TiOz sol was determined to be -350 nm (Figure 1). Solar radiation has a small fraction of energies below this absorption edge. Thus, one could expect the possibility of using solar radiation for the photocatalytic degradation of ferricyanide in the presence of titania sol. To test this hypothesis, a series of experiments were carried out using solar radiation instead of the 4-W mercury lamp as the W source. A typical experiment consisted of preparing a stabilized TiOz sol with a 1mM ferricyanide solution, as in the previous case. A reference solution of the same concentration of ferricyanide but no titania sol was also prepared. Both were simultaneously exposed to sunlight and periodically the concentrations of ferricyanide in both solutions were measured spectrometrically. The results of typical experiments carried out at pH = 10 are presented in Table 11. It is reasonable to assume that under similar experimental conditions free cyanides will also be degraded to the nontoxic products. Studies are underway to elucidate the reaction mechanism and the rates of degradation in this titania sol catalyzed photodegradation of complex cyanides. In conclusion, the results of the present investigation show that the photocatalyzed degradation of cyanide wastes (both complexed and free cyanides) with UV-irradiated titania semiconductor sol can be developed into an alternate cyanide waste treatment process. It could also
0 1992 American Chemical Society
Environ. Sci. Technol., Vol. 26,No. 3, 1992 825
particles can be easily coagulated and removed after the degradation reaction. The precipitated titania can be recycled to make fresh titanium tetrachloride. But further work is needed before this can become a reality. Registry No. Fe(CN)G.3K,13746-66-2; TiOz, 13463-67-7.
Literature Cited
Flgure 1. Photocatalytic degradation of ferricyanide in the presence of titania sol.
be developed as an extension of the presently used alkaline chlorination process to degrade the complexed cyanides that are not affected by chlorination. Under certain conditions, inexpensive solar radiation can be used as a UV source. Titania sol photocatalyzed degradation of cyanide wastes is environmentally friendly. The process does not add any toxic pollutant. TiOz is nontoxic and the colloid
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Environ. Sci. Tachnol., Vol. 26, No. 3, 1992
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i.,
Received for review October 14, 1991. Accepted December 10, 1991.
